<?php
  function authenticate() {
    header('WWW-Authenticate: Basic realm="Test Authentication System"');
    header('HTTP/1.0 401 Unauthorized');
    echo "You must enter a valid login ID and password to access this resource\n";
    exit;
  }

  if (!isset($_SERVER['PHP_AUTH_USER']) ||
      ($_POST['SeenBefore'] == 1 && $_POST['OldAuth'] == $_SERVER['PHP_AUTH_USER'])) {
   authenticate();
  }
  else {
   echo "<p>Welcome: {$_SERVER['PHP_AUTH_USER']}<br />";
   echo "Old: {$_REQUEST['OldAuth']}";
   echo "<form action='{$_SERVER['PHP_SELF']}' METHOD='post'>\n";
   echo "<input type='hidden' name='SeenBefore' value='1' />\n";
   echo "<input type='hidden' name='OldAuth' value='{$_SERVER['PHP_AUTH_USER']}' />\n";
   echo "<input type='submit' value='Re Authenticate' />\n";
   echo "</form></p>\n";
  }